Some ultrasonic sensors that detect objects or measure distances by transmitting and receiving ultrasonic waves using ultrasonic transducers each use one ultrasonic element as an ultrasonic element for transmission and an ultrasonic element for reception in order to reduce the ultrasonic sensors in size. The frequencies of the ultrasonic waves to be transmitted and received are set to the resonant frequencies of the ultrasonic transducers in the ultrasonic sensors and electrical pulse signals of the resonant frequencies (including frequencies approximating to the resonant frequencies) are applied to the ultrasonic transducers. In this case, a phenomenon called reverberation in which ultrasonic vibration mechanically continues for a short time occurs even after the application of the electrical pulse signals is stopped. When one ultrasonic transducer is used as the ultrasonic element for transmission and the ultrasonic element for reception, the reverberation can prevent the detection from the received ultrasonic waves.
Patent Document 1 discloses an ultrasonic sensor intended to reduce the duration of the reverberation. The ultrasonic sensor described in Patent Document 1 is configured so as to, immediately after a driving signal to be intermittently supplied to the ultrasonic transducer is stopped, supply a reverberation suppression signal having a phase opposite to the phase of the driving signal to the ultrasonic transducer only during a time period corresponding to the half period of the driving signal to drive the ultrasonic transducer. Accordingly, the vibration is cancelled only during the time period corresponding to the half cycle of the driving signal immediately after the ultrasonic transducer is driven to reduce the duration of the reverberation.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-146121
Inductance components other than the ultrasonic transducer, for example, inductance components in a step-up transformer are included in the circuit of each ultrasonic sensor. Since the other inductance components are not considered in the ultrasonic sensor described in Patent Document 1, there are cases in which the reverberation suppression signal is affected by the effect of the other inductance component and so on to prevent the vibration of the ultrasonic transducer from being sufficiently cancelled.